使用RxJava从异步调用返回值？

Question

Okay, so what I'm trying to do is collect a list of user_uid's from firebase then for each user_uid fetch an image url and combine it and return one Object. The problem I'm running into is returning the Object from the asynchronous call. I'm trying to implement rxjava to achieve this, but I'm still getting empty Objects. below I'll have code and database structure so you can see what I'm trying to achieve, thank you all in advanced for any help

this is the database structure:

Asynchronous call:

private Note createNoteWithArtists(Note note, ArrayList<String> user_uids) {
    DatabaseReference db_users = db_root.child("Users");
    ArrayList<User> artists = new ArrayList<>();
    Note note = note;
    for (String each_uid : user_uids) {
        db_users.child(each_uid).addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                String uid = dataSnapshot.getKey();
                String image = dataSnapshot.child("image").getValue(String.class);
                String username = dataSnapshot.child("username").getValue(String.class);
                User user = new User(username, null, image, uid);
                artists.add(user);
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });
    }
    note.setArtists(artists);
    return Note;
}

attempting to use RxJava:

io.reactivex.Observable<Note> observable = io.reactivex.Observable.create(subscriber -> {
                subscriber.onNext(createNoteWithArtists(note, artists));
                subscriber.onComplete();
            });
            observable.subscribe(note -> loadNoteResults.NoteLoaded(note, true)).dispose();

It returns the note, but the nested artists object is still empty :/

Answer 1

Observable.create(...) will emit a Note as soon as createNotWitArtists returns a value. Your RxJava attempt is functionally equivalent to:

Note emptyNote = createNoteWithArtists(note, artists);
loadNoteResults.NoteLoaded(emptyNote , true);

Take a look at this approach:

private Single<User> getUser(String user_uid) {
    DatabaseReference db_users = db_root.child("Users");
    return Single.create(emitter -> {
        db_users.child(each_uid).addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                String uid = dataSnapshot.getKey();
                String image = dataSnapshot.child("image").getValue(String.class);
                String username = dataSnapshot.child("username").getValue(String.class);
                User user = new User(username, null, image, uid);
                emitter.onSuccess(user);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                emitter.onError(databaseError);
            }
        });
    });
}

The idea is to use Single.create that emits when each onDataChange is called.

Then you can combine all Single s using toList() like this:

// Note note
// List<String> artist

Observable.fromIterable(artistIds) // Observable<String>
    .flatMapSingle(this::getUser) // Observable<User>
    .toList() // Single<List<User>>
    .subscribe( artists -> {
        note.setArtists(artists);
        loadNoteResults.NoteLoaded(note, true);
    });

Also, don't forget to put schedulers whenever it is needed.